There are many types of Unmanned Aerial Vehicles (UAVs), which are also known as drones. Types of UAV vehicles include multirotors, small hand thrown fixed-wing planes, medium sized vehicles that can be catapult launched or take off from short runways or very large vehicles that can fly around the world conducting reconnaissance missions and launching missiles. A new breed of vehicle called UCAVs for Unmanned Combat Air Vehicles can take off and land on aircraft carriers.
One of the main challenges for UAVs is flight duration. Typical UAVs use batteries or internal combustion engines for propulsion. Batteries are extremely heavy for driving propellers with electric motors, and internal combustion engines are very inefficient at converting aviation fuels (hydrocarbon based) to drive propellers mechanically thus limiting flight duration. A more efficient propulsion system is one that uses hydrogen fuel that is light weight and has the highest stored energy content per unit mass compared to other fuels. By using a very energy efficient fuel cell to convert the hydrogen into electricity to drive the electric motors, extreme durations can be achieved. Table 1 is a comparison of flight duration for the various power systems as modeled for the same aircraft. The power system with the greatest duration is a fuel cell with liquid hydrogen storage.
TABLE 1Power/Energy Source for 12 ft. Wing Span UAVFlight DurationBatteries<1hourInternal Combustion Engine with Aviation Fuel3hoursFuel Cell with Gaseous Hydrogen Storage13hoursFuel Cell with Liquid Hydrogen Storage>30hours
Liquid hydrogen has a density of 70 kg/m3 when stored at 21 Kelvin at 1 atmosphere of pressure. This is a density increase of 2.8 times compared to compressed hydrogen gas storage at 350 bar (5,100 psi) at ambient temperature. Liquid hydrogen is vaporized and warmed using ambient temperatures and a heat exchanger and then is consumed by the fuel cell to make electricity and water, which is released over-board. The fuel cell is typically a Proton Exchange Membrane (PEM) fuel cell, which operates at around 60° C.
A liquid hydrogen powered UAV needs to have fuel transferred into the UAV liquid hydrogen tank from a storage dewar. The transfer process starts by pressurizing the storage dewar with warm gaseous hydrogen also called autogenous pressurization. The warm gaseous hydrogen comes from withdrawing some of the stored liquid and warming it up in a vaporizer. The gas space above the liquid (ullage space) is then pressurized. Pressurization of the storage or supply dewar can be conducted using a gas other than hydrogen such as helium that does not condense in the liquid, which is called non-condensable pressurization. A liquid supply valve is then opened and the pressure pushes the liquid out of the storage dewar and into the UAV liquid hydrogen tank, which is vented to atmospheric pressure during the filling process. Alternatively, a pump can be used to transfer the liquid hydrogen. A comparison of these two prior art processes for transferring liquid hydrogen are shown in FIG. 1.
The transfer equipment used includes transfer lines that are vacuum jacketed and are connected to the dewars with bayonet fittings. The transfer of liquid hydrogen is currently a manual process that involves many hands-on steps. These steps include: physically connecting up transfer equipment such as hoses, flanges, fittings, and bayonets; and conducting flow or pressure purges of the system prior to and after the transfer process in order to maintain cleanliness and to mitigate the generation of combustible mixtures of air and hydrogen. The purges involve connecting up the purge gas source, opening and closing valves, and monitoring pressures per specific pre-determined values based on the volume of the system being purged. In the case of flow purges the time of the flow process is measured based on the volume of the system and the flow rate of the purge gas. The flow rate of the purge gas is measured either by the pressures across the flow valve or a flow measuring device. Vacuum purges may also be done, which require the use of a vacuum pump. The vacuum pump hose is connected to the pump-out port and the vacuum level is measured via a thermocouple bulb or a variety of different vacuum gauges suitable for the vacuum range specified for the purge. The vacuum pump then may need to be disconnected from the system.
When the liquid hydrogen transfer lines get cold, moisture or residual gases will condense and potentially freeze on the cold surfaces. Helium gas is typically used as a purge gas because it has a lower condensation point than liquid hydrogen and is thus called a noncondensable gas in the presence of liquid hydrogen.